A magnetic memory, especially a magnetic random access memory (MRAM) operates as a nonvolatile memory capable of a high-speed operation and rewriting an infinite number of times. Therefore, some types of MRAMs have been put into practical use, and some types of MRAMs have been developing to improve their general versatility. In the MRAM, a magnetic material is used as a memory element, and data is stored in the memory element as a magnetization direction of the memory element. Some methods for switching the magnetization direction of the memory element are proposed. Those methods have in common with usage of a current. To put a MRAM into practical use, it is important to reduce the writing current as much as possible.
According to the non-patent literature 1, it is required that the wiring current should be reduced to be equal to or less than 0.5 mA, preferably equal to or less than 0.2 mA. This is because the minimum layout can be applied to the 2T-1MTJ (Two transistors-One Magnetic tunnel junction) circuit configuration proposed in the non-patent literature 1 to realize the cost performance equal to or more than that of the existing volatile memory.
The most general method of writing data in a MRAM is to switch a magnetization direction of magnetic memory element by a magnetic field which is generated by passing a current through a wiring line for a writing operation prepared on the periphery of the magnetic memory element. Since this method uses a magnetization switching caused by the magnetic field, the MRAM can theoretically perform writing at a speed of 1 nano-second or less and thus, the MRAM is suitable for a high-speed MRAM. However, a magnetic field for switching magnetization of a magnetic material securing thermal stability and resistance against external disturbance magnetic field is generally a few dozens of [Oe]. In order to generate such magnetic field, a writing current of about a few mA is needed. In this case, a chip area is necessarily large and power consumed for writing increases. Therefore, this MRAM is not competitive with other kinds of random access memories. In addition, when a size of a memory cell is miniaturized, a writing current further increases and is not scaling, which is not preferable.
Recently, as methods to solving these problems, following two methods are proposed. The first method is a method using a spin transfer magnetization switching. This method uses a laminated layer including a first magnetic layer (magnetization free layer) which has magnetization that can be switched, and a second magnetic layer (reference layer) which is electrically connected to the first magnetic layer and has magnetization that is fixed. In the method, the magnetization in the first magnetic layer (magnetization free layer) is switched by using an interaction between spin-polarized conduction electrons and localized electrons in the first magnetic layer (magnetization free layer) when a current flows between the second magnetic layer (reference layer) and the first magnetic layer (magnetization free layer). A reading operation is carried out by using a magnetoresistive effect generated between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer). Therefore, the MRAM using the spin transfer magnetization switching method is an element having two terminals.
The spin transfer magnetization switching is generated when a current density is equal to or more than a certain value. Accordingly, as the size of the element decreases, the writing current is also reduced. In other words, the spin transfer magnetization switching method is excellent in scaling performance. However, generally, an insulating film is provided between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer) and a relatively large current should be made to flow through the insulating film in the writing operation. Thus, there are problems regarding resistance to writing and reliability. In addition, there is concern that a writing error occurs in the reading operation because a current path of the writing operation is the same as that of the reading operation. As mentioned above, although the spin transfer magnetization switching method is excellent in scaling performance, there are some obstacles to put it into practical use.
On the other hand, the second method, which is a magnetization switching method using a current induced domain wall motion effect, can solve the above-mentioned problems that the spin transfer magnetization switching method is confronted with. For example, a MRAM using the current induced domain wall motion effect is disclosed in the patent literature 1. That is, the patent literature 1 discloses a magnetic memory apparatus and a method of writing a magnetic data. The magnetic memory apparatus includes a magnetization fixed layer, a tunnel insulating layer, a magnetization free layer and a pair of magnetic data writing terminals. The magnetization fixed layer has fixed magnetization and conductive. The tunnel insulating layer is laminated on the magnetization fixed layer. The magnetization free layer includes a connection portion laminated on the magnetization fixed layer through the tunnel insulating layer, domain wall pinning portions formed at both ends of the connection portion, and a pair of magnetization fixed portions adjacent to the domain wall pinning portions and having fixed magnetization opposite to each other. The pair of magnetic data writing terminals is electrically connected to the pair of magnetization fixed portions, and makes a current flow through the connection portion, the pair of domain wall pinning portions and the pair of magnetization fixed portions of the magnetization free layer. In the first magnetic layer (magnetization free layer) having the magnetization which can be switched of the above MRAM using the current induced domain wall motion effect, generally, magnetization of both end portions are fixed such that the magnetization of one end portion is approximately anti-parallel to that of the other end portion. In the case of such magnetization arrangement, a domain wall is introduced into the first magnetic layer. Here, as reported in the non-patent literature 2, when a current flows through the domain wall, the domain wall moves in the direction same as the direction of the conduction electrons. Therefore, the data writing can be realized by making the current flow inside the first magnetic layer (magnetization free layer). The data reading is realized by using the magnetoresistive effect caused by a magnetic tunnel junction provided in a region where the domain wall moves. Therefore, the MRAM using the current induced domain wall motion method is an element having three terminals, and fits in the 2T-1MTJ configuration proposed in the above-mentioned non-patent literature 1.
The current induced domain wall motion is generated when the current density is equal to or more than a certain value. Thus, this MRAM has the scaling property similar to the MRAM using the spin transfer magnetization switching. In addition, in the MRAM element using the current induced domain wall motion, the writing current does not flow through the insulating layer in the magnetic tunnel junction and the current path of the writing operation is different from that of the reading operation. Consequently, the above-mentioned problems caused in the spin transfer magnetization switching can be solved.
However, in the non-patent literature 2, a current density of approximately 1×108 A/cm2 is required for the current induced driven domain wall motion. For example, it is assumed that a width and a thickness of a layer where the domain wall motion arises are 100 nm and 10 nm, respectively. In this case, the writing current is 1 mA. This cannot satisfy the above-described condition for the writing current. However, as described in the non-patent literature 3, it is reported that, by using a material having perpendicular magnetic anisotropy as a ferromagnetic layer (magnetization free layer) where the current induced domain wall motion arises, the writing current can be sufficiently reduced. Because of this, in the case of manufacturing an MRAM using the current induced domain wall motion, it is preferable to use a ferromagnetic material having perpendicular magnetic anisotropy as a layer (magnetization free layer) where the domain wall motion arises.
As a related technique, the patent literature 2 discloses a varying method of a magnetization state of magnetoresistive effect element using a domain wall motion, a magnetic memory element and a solid magnetic memory using the method. The magnetic memory element includes a first magnetic layer, an interlayer and a second magnetic layer. The magnetic memory element records data as magnetization directions of the first and second magnetic layers. The magnetic memory element records data by forming regularly magnetic domains with mutual anti-parallel magnetizations and a domain wall separating those magnetic domains in at least one of the magnetic layers and moving the domain wall in the magnetic layer so as to control positions of the adjacent two magnetic domains.
The patent literature 3 discloses a magnetoresistive effect element based on a domain wall motion using a pulse current and a high-speed magnetic recording device. This magnetoresistive effect element includes a first magnetization fixed layer/a magnetization free layer/a second magnetization fixed layer. The magnetoresistive effect element includes a mechanism for inducing a domain wall generation in a transition region between the magnetization fixed layer and magnetization free layer, the transition region being at least one of a boundary between the first magnetization fixed layer/the magnetization free layer and a boundary between the magnetization free layer/the second magnetization fixed layer. The magnetization directions of these magnetization fixed layers are set to approximately anti-parallel magnetizations. The domain wall exists one of the transition regions between the first magnetization fixed layer/the magnetization free layer and between the second magnetization fixed layer/the magnetization free layer. By applying a current less than 106 A/cm2 with a certain pulse width, the domain wall moves between two transition regions, thereby making the magnetization of the magnetization free layer switch and detecting the magnetoresistive value caused by the switching of the relative magnetization direction.